PCI Speed and Overclocking: A Closer Look at A64 and P4 Chipsets

PCI Speed and Overclocking: Test Results

PCI Bus Overclocking Tests
Motherboard CPU	Chipset	PCI Speed at 220 Base FSB Setting	Functional PCI Lock
Asus P4C800-E Pentium 4 3.2C	Intel 875p	33.3	YES
Asus P4S800D-E Pentium 4 3.2C	SiS 655TX	33.3	YES
Soltek SL-PT880PRO Pentium 4 3.2C	VIA PT880	33.3	YES
AOpen AK86-L Athlon 64 3200+	VIA K8T800	36.6	NO
ECS 755A2 Athlon 64 3200+	SiS 755	36.6	NO
Soyo CK8 Dragon Plus Athlon 64 3200+	nForce3-150	36.6	NO

As you will see in results with the new VIA PT880 board for Intel, early testing raised questions about whether the PCI/AGP lock was working. However, tests with PC Geiger showed the new VIA PT880, as well as the Intel 875P and SiS 655TX, all have a working PCI lock. We have seen outstanding overclocking results with the Intel chipsets and SiS 655TX chipsets; the working PCI lock certainly contributes to that outstanding overclocking performance.

The big surprise here is that none of the current chipsets for Athlon 64 have a working PCI lock, whether they claim one or not. This was a complete surprise and our first inclination was to throw out the results, since speeds on the Athlon 64 are derived in a very different way with Hypertransport. However, oscilloscope results and tests elsewhere pretty much support our findings. There does not appear to be a working PCI lock on any current chipsets for Athlon 64. Certainly, this contributes to the poorer overclocking that we have seen on A64 inasmuch as on-board component frequencies and AGP bus are related to PCI bus.

Please keep in mind that we have only measured PCI bus speed with PC Geiger. While we expect on-board component speeds and AGP to be affected also, we cannot say that with certainty on Athlon 64 systems. Some reports on the web claim that AGP frequency is fixed on both VIA and nVidia chipsets on the Athlon 64. We can neither confirm nor deny these claims with the simple tests that we have performed here. Also keep in mind that results are for 6 individual motherboards. We can generally assume these results apply to the chipset used, but in some cases, this may be a stretch. The ECS 755A2 is designed to be a value board, for example, and 'PCI lock' may simply not have been implemented on this board. On VIA and nVidia chipsets for Athlon 64, our results do mirror those measured objectively at other sites using other VIA and nVidia A64 boards and we are more confident applying these results to chipset assumptions.

We will continue to look at PCI speeds in future tests of new boards and chipsets, particularly those designed for Athlon 64 processors. For now, it certainly appears that a floating PCI bus is one of the factors holding back overclocking on all current Chipsets for the Athlon 64.